This invention relates to interference pigments, a method for manufacture thereof, effect paint formulated with interference pigments, and paintwork applied therewith.
This application claims the priority of German patent application No. 19619973.5-43, filed May 17, 1996, the disclosure of which is expressly incorporated by reference herein.
In the effort to obtain brilliant or effect-filled color impressions, especially in motor vehicle bodies, so-called effect paints have been developed. EP 383 376 A1 describes such an effect paint in which small mica platelets are coated uniformly on all sides with a cross-linked liquid crystalline polymer (LCP) in a chiral-nematic arrangement of the mesogens. The color appearance of such pigments results from an interference phenomenon. Those light waves in the incident light whose wavelengths interfere with the equidistant lattice plane spacing of the liquid crystalline polymer are reflected from the LCP layer, while light components of other wavelengths pass through the transparent paint body, and are partly reflected from the surface of the mica platelets and partly absorbed. Since light absorption is only slight because of the transparency of the mica platelets, the interferential color effect obtained with this type of interference pigment is determined primarily by the absorption effect of the primer; more detail on this is provided below. In addition, uniform and complete coating of small mica particles with an LCP coating without the pigments clumping in the process is very expensive, making effect paint and therefore effect paintwork on the object in question quite expensive.
For example, the manufacture of interference pigments and liquid crystal polymers is known from EP 601 483 B1 (corresponding to DE 42 40 743 A1), DE 44 18 075 A1, or DE 44 18 076 A1, these polymers creating a more intensive interferential color effect and also being easier to manufacture. These interference pigments consist as a whole of small pieces of a thin, crosslinked, colorless, clear transparent film of liquid crystalline polymers. The color effect that can be achieved with them is based on the regular structure and uniform arrangement of the molecules in the shape of a liquid crystal and on the resultant interference with a certain portion of the light spectrum in which the pigment produces a reflecting effect. The other light components pass through the pigment. As a result, striking color effects of different kinds can be produced, depending on the structure of the paintwork and/or the interference pigments and their mixtures. The basic shade of the interference pigments is determined by the color impression, or the color that is seen when the painted surface is illuminated perpendicularly and viewed perpendicularly. When the beam path is directed at an angle to the surface, the lattice plane intervals, due to the geometry, appear to be changed relative to the orthogonal beam direction, so that the color impression shifts toward another color that is offset toward shorter wavelengths in the color spectrum, and this depends on the relative viewing direction of the surface. Depending on the position of a certain part of the surface relative to the line of sight of the viewer, the surface portion appears in the basic color or in the other color with the shorter wavelength. For example, interference pigments with a basic red color can change to green; a color change between green and blue can be produced using different interference pigments.
The intensity of the colors that can thus be perceived increases with the darkness of the background color on which the paint layer that determines the color rests, with the color of the background however having to be determined by color pigments that have an absorptive effect. This is due to the fact that the light components that pass through the interference pigments are absorbed more or less completely by the dark background to a degree that increases with the darkness of the background.
The interference pigments of the type under discussion here are obtained by the polymers being spread in the liquid or liquid crystalline state onto a smooth substrate, a polished roller for example, forming a thin film. The spreading process produces an alignment of the molecules within the film, namely a homogeneous orientation; it is only because of this alignment that the color exhibits an interference color. During the shearing of the liquid crystalline polymers, equidistant lattice planes automatically form during spreading and thus create color selective deflecting structures.
Known interference pigments, as we have said, are clear and transparent and therefore do. not have any covering power. They therefore require a uniformly colored background in order to produce a uniform color impression. On the other hand, the color of the background can change within a wide range of workpieces, for example when filler paint from different manufacturers is used, with said paints differing in color, or when differently primed structural or body parts are processed on one and the same workpiece. Prior to the application of an interference effect paint that has no covering power, therefore, assurance must be provided that not only all of the parts of the workpiece but also all of the workpieces within a series are painted uniformly with only one primer whose color is specified, necessitating an additional painting and baking process. In the case of repairs as well, the body part whose paint is to be repaired or even the entire body must first be primed uniformly and in exactly the same color as originally and this primer must then be baked. Only then can the effect paint be applied to the repair. This is costly and awkward for routine applications but especially for repairs. Theoretically, it would be possible when formulating an effect paint to mix absorption pigments into the paint along with interference pigments in order to make the paint exhibit covering power as a result. However, to achieve a good covering power, such a large amount of absorption pigment would have to be added to the paint that the effect of the interference pigments would be very sharply attenuated as a result because they are dependent on good transparency of the body of the paint. If only a small amount of absorption pigments is mixed in with the paint and adversely affects the interferential color effect of the interference. pigments only. slightly,. the paint, in order to achieve a sufficient covering power, would have to be applied in a layer that was so thick that it could not be applied in a single step. Such a paint would have to be applied in a minimum of two steps, which would offer no advantages over a previous application of a primer with good covering power.